feat(map): add peak highlights and live metrics to PNG export

- Draw rings around hottest and coldest sensors on export.
- Extend export_image to support second line for live metrics.
- Format detailed sensor stats in export footer.
- Add test coverage for footer rows, overlays, and stats.

src/pygui/ISC/Tabs/components/ReadoutPanel.qml

@@ -15,7 +15,7 @@ ColumnLayout {
     property alias heatmapBlend: heatmapSlider.value
     property alias showThickness: thicknessToggle.checked
 
-    signal exportRequested(string filePath)
+    signal exportRequested(string filePath, string extra)
 
     component PanelCheckBox: CheckBox {
         id: toggle
@@ -80,8 +80,8 @@ ColumnLayout {
 
     StreamControlPanel {
         Layout.fillWidth: true
-        onExportRequested: function(filePath) {
-            root.exportRequested(filePath);
+        onExportRequested: function(filePath, extra) {
+            root.exportRequested(filePath, extra);
         }
     }